The present invention relates to guides to assist connection of a pipe, e.g. a conductor pipe, lowered from a platform over the surface of the sea, to a wellhead positioned on the sea bed and to processes of connection employing such guides.
Offshore oil and gas fields have been developed using large platforms, which were constructed and installed offshore before drilling began. This meant that production rate would increase gradually as each well was drilled and completed.
In order to reduce the time taken to produce hydrcarbons at a significant rate, it is possible to drill wells using mobile drilling rig during the period required to construct the production platform. When the platform has been accurately installed over the "pre-drilled" wells, conductor pipes are lowered through guides to connect to the seabed wellheads. These pipes effectively lengthen the well bores above water level, so that the wellheads can be positioned at platform deck level. This permits tubing to be installed from the deck, and wellhead valves which form the "Christmas tree" can be manually actuated by an operator on the platform. These pipes are known as "tie-back" conductor pipes.
As offshore fields progress to deeper water, mobile rigs have been developed from being bottom supported, or "jack-up" type, to floating rigs which are either shipshape or semi-submersible. Floating rigs are less easily maintained directly over a point on the sea bed than jack-up rigs, as they tend to drift laterally against their anchor chains or their dynamic positioning reference beacon.
Drilling from a jack-up rig involves the use of a conductor pipe extending from the sea bed to the deck level where a safety system or "blow-out-preventor (BOP) is mounted on top of the conductor. The BOP is used to control the well in the event that the drill bit pierces a formation containing pressurised oil or gas which is at a higher pressure than that exerted by the head of drilling mud in the hole. With a floating rig the lateral motion of the vessel is accommodated by a flexing "riser" incorporating an articulated joint at the lower end.
Because the articulated joint is relatively poor at retaining high pressure oil and gas, the BOP is mounted on a sea bed wellhead.
This difference in drilling method means that the vertical misalignment tolerance of the wellbore immediately below the seabed is wider for wells drilled from a floating rig than those drilled from a jack-up rig. Also in deeper water, the fixed platform structure tends to an increasing height to base-width ratio, and therefore vertically becomes less controllable. If "pre-drilling" is to be successfully applied in deeper water, the connection system between the conductor pipes and the sub sea wellheads must be able to accept a wider angular misalignment tolerance between guide funnels in the platform structure and the sub sea wellbores.